1. Field of the Invention
The invention relates to the field of biomedical imaging, and in particular to functional optical coherence tomography and functional optical Doppler tomography.
2. Description of the Prior Art
Direct visualization of physiological anatomy provides important information to the diagnostician and therapist for the evaluation and management of disease. High spatial resolution noninvasive techniques for imaging in vivo blood flow dynamics and tissue structure are currently not available as a diagnostic tool in clinical medicine. Such techniques could have a significant impact for biomedical research and clinical diagnosis. Techniques such as Doppler ultrasound and laser Doppler flowmetry (LDF) are currently used in medical diagnosis for blood flow velocity determination. Doppler ultrasound uses the principle that the frequency of ultrasonic waves backscattered by moving particles are Doppler shifted. However, the relatively long acoustic wavelengths required for deep tissue penetration limits the spatial resolution to approximately 200 μm. Although LDF has been used to measure mean blood perfusion in the peripheral microcirculation, strong optical scattering in biological tissue limits spatially resolved flow measurements by LDF.
Optical Doppler tomography (ODT), also named Doppler optical coherence tomography (Doppler OCT), is capable of measuring microflows using the optical Doppler effect. Early ODT systems were unable to achieve high imaging speed, high velocity sensitivity and high spatial resolution simultaneously. A phase-resolved algorithm was developed in the prior art to obtain high velocity sensitivity while maintaining high imaging speed and high spatial resolution. This technique has been applied to clinical investigations and microfluidic study. To further the study of microflows, a Doppler variance algorithm has been added to the phase-resolved ODT. The Doppler frequency shift depends on the Doppler angle between the probe and the flow direction. By contrast, Doppler variance is less sensitive to Doppler angle and is more efficient for mapping the flows buried in non-transparent media. Currently, ODT systems are implemented in the time domain. Although real-time 2-D flow imaging has been achieved with the time domain ODT, 3-D mapping of complex flows in microfluidic networks requires even higher speed and better sensitivity. In the time domain ODT, mechanical devices are required for axial scanning (A-line scanning) and limit the imaging speed and velocity dynamic range.
Recently, frequency domain F-OCT or FDOCT has shown advantages in imaging speed and signal-to-noise ratio over the time domain OCT. Since the velocity dynamic range of a phase-resolved ODT system is determined by A-line scanning rate, it would be advantageous to extract Doppler information using the frequency domain method. The measurement of flow profiles has been demonstrated using frequency domain method, but the Doppler variance tomography has not been performed in frequency domain.
In Fourier domain OCT (FDOCT) DC and autocorrelation noises decrease the system sensitivity and the mirror image due to the Fourier transformation limits the imaging range of FDOCT. Several methods have been developed to resolve these problems. Phase retrieval algorithms using five interferograms with defined phase relations or a 90° phase shift introduced by translation of the reference mirror were adopted to obtain complex signals to cancel out the autocorrelation noise terms as well as the DC signal. However, these non-instantaneous algorithms require high stability of the systems and limit the imaging speed due to mechanical translation. The phase shift of an N by N (N>2) fiber coupler has been proposed to access the complex image but it has the drawback of phase drift due to temperature sensitivity of the coupler splitting ratio. What is needed is a method which can achieve a full range complex signal to eliminate DC and autocorrelation noises as well as the mirror image.